java - 使用流的 map 收集产生奇怪的编译错误

Question

我有这个方法:

 public void fields(TableField... s){
    // compilation error on next line
    Collection<String> fields = Arrays.asList(s).stream().map(v -> v.getValue());
    this.fields.addAll(fields);
  }

TableField 看起来很简单:

class TableField {
  public String getKey() {
    return this.key;
  }

  public String getValue() {
    return this.value;
  }
}

但我看到了这个编译错误:

Incompatible types. Required Collection but 'map' was inferred to Stream: no instance(s) of type variable(s) R exist so that Stream conforms to Collection

Answer 1

map 返回转换后的 Stream。如果你想要一个集合，你需要收集流。例如:

Collection<String> fields =
    Arrays.asList(s).stream().map(v -> v.getValue()).collect(Collectors.toList());